1. Field of the Invention
The present invention relates to a scroll pump and, in particular, to a pump head assembly of a scroll pump which includes plate scrolls having nested scroll blades, and a tip seal(s) that provides a seal between the tip of the scroll blade of one of the plate scrolls and the plate of the other plate scroll. The present invention also relates to a method of calibrating a scroll pump in either the assembling of the pump or as part of a trouble shooting or maintenance operation.
2. Description of the Related Art
A scroll pump is a type of pump that includes a stationary plate scroll having a spiral stationary scroll blade, and an orbiting plate scroll having a spiral orbiting scroll blade. The stationary and orbiting scroll blades are nested with a clearance and predetermined relative angular positioning such that a pocket (or pockets) is delimited by and between the scroll blades. The scroll pump also has a frame to which the stationary plate scroll is fixed and an eccentric drive mechanism supported by the frame. These parts generally make up an assembly that may be referred to as a pump head (assembly) of the scroll pump.
The orbiting plate scroll and hence, the orbiting scroll blade, is coupled to and driven by the eccentric driving mechanism so as to orbit about a longitudinal axis of the pump passing through the axial center of the stationary scroll blade. The volume of the pocket(s) delimited by the scroll blades of the pump is varied as the orbiting scroll blade moves relative to the stationary scroll blade. The orbiting motion of the orbiting scroll blade also causes the pocket(s) to move within the pump head assembly such that the pocket(s) is selectively placed in open communication with an inlet and outlet of the scroll pump.
In an example of such a scroll pump, the motion of the orbiting scroll blade relative to the stationary scroll blade causes a pocket sealed off from the outlet of the pump and in open communication with the inlet of the pump to expand. Accordingly, fluid is drawn into the pocket through the inlet. Then the pocket is moved to a position at which it is sealed off from the inlet of the pump and is in open communication with the outlet of the pump, and at the same time the pocket is collapsed. Thus, the fluid in the pocket is compressed and thereby discharged through the outlet of the pump. The sidewall surfaces of the stationary orbiting scroll blades need not contact each other to form a satisfactory pocket(s). Rather, a minute clearance may be maintained between the sidewall surfaces at the ends of the pocket(s).
A scroll pump as described above may be of a vacuum type, in which case the inlet of the pump is connected to a chamber that is to be evacuated.
Furthermore, oil may be used to create a seal between the stationary and orbiting plate scroll blades, i.e., to form a seal(s) that delimits the pocket(s) with the scroll blades. On the other hand, certain types of scroll pumps, referred to as “dry” scroll pumps, avoid the use of oil because oil may contaminate the fluid being worked by the pump. Instead of oil, dry scroll pumps employ a tip seal or seals each seated in a groove extending in and along the length of the tip (axial end) of a respective one of the scroll blades (the groove thus also having the form of a spiral). More specifically, each tip seal is provided between the tip of the scroll blade of a respective one of the plate scrolls and the plate of the other of the plate scrolls, to create a seal which maintains the pocket(s) between the stationary and orbiting scroll blades. Further in this respect, scroll pumps of the type described above typically require a certain degree of axial compliance among respective parts of the pump head assembly to maintain an effective seal between the opposing scroll blades and plates.
In general, there are two types of tip seal arrangements to meet these requirements: energized and non-energized. An energized type of tip seal arrangement includes a tip seal seated in the tip of the scroll blade of one of the plate scrolls, and a spring that biases the tip seal against the plate of the other of the plate scrolls. A typical non-energized type of tip seal arrangement has only a solid plastic tip seal seated in the tip of the scroll blade of one of the plate scrolls and the solid plastic tip seal directly confronts the plate of the other of the plate scrolls.
In the energized type of tip seal arrangements, the tip seals are continuously worn because they are constantly biased by a positive spring force into engagement with the opposing scroll plate. As a result, spring-biased tip seals must be replaced rather frequently. The solid plastic tip seals of the non-energized arrangements have a relatively longer useful life than the conventional spring-biased tip seals. However, the use of solid tip seals presents its own set of problems.
For instance, the tolerances of dimensions of various components of scroll pumps that employ non-energized tip seals must be maintained within narrow ranges to ensure proper sealing of the tip seal without excessive compression of the seal. More specifically, in a compressor type of scroll pump, forces generated by the compressed gas act on the tip seals to force them towards the opposing plate, i.e., to in effect energize the tip seals. However, in a vacuum type of scroll pump, the tip seals operate in an environment of minimal absolute pressures. Therefore, there is little, if any gas pressure to energize the tip seals, especially at the outer wraps of the scroll blades where the greatest vacuum levels exist. Accordingly, the axial dimensions and alignment of parts constituting the head assembly must be precise to ensure that any gaps between the solid tip seals and the opposing scroll plates are minimal. If, on the other hand, the tip seals are compressed too much between the scroll blades and the opposing scroll plates, the resulting friction and heat can overload and damage parts of the pump such as the bearings of the drive mechanism.
In consideration of these potential problems in a vacuum type of scroll pump, a prior art technique uses shims to control the relative axial positions of components of the head assembly of the pump. For example, a shim having a thickness of 0.001″ may be inserted into a bearing train of the drive mechanism to reduce the gap between the tip seals and the opposing scroll plates by 0.001″. However, if the use of this particular shim does not result in a satisfactory performance and/or gives rise to excessive friction, the pump has to be disassembled and the shim has to be replaced with a shim of a different thickness. Then the pump has to be re-assembled and tested again. Thus, this trial and error technique may be onerous and time consuming.